1. Field of the Invention
The invention relates in general to a projecting device, and more particularly to a projecting device having a discharge lamp.
2. Description of the Related Art
Discharge lamps have a large variety and are categorized into high-voltage mercury lamp, metal halogen lamp, low-voltage sodium lamp, and high-voltage sodium lamp. Take the high-voltage mercury lamp for example. The high-voltage mercury lamp illuminates by using a high voltage to form a strong electric arc between two electrodes and excite the gas ionization of the mercury or mercury compound inside the discharge vessel. The mercury atoms absorb the energy of the electrons and convert into electromagnetic waves radiated out. When the density of mercury is high enough, the radiated electromagnetic waves is re-absorbed by mercury atoms for jumping to greater energy level and visible light is released.
Conventional high-voltage mercury lamp uses a higher voltage of about 20 KV to ignite the lamp first, and then uses a lower voltage of about 70V to maintain the operation of the lamp. During the ionization process, electrons would be neutralized with the ions, and once the speed rate of the ions generation is smaller than that of ions neutralization, discharge process would not occur. Under cold lamp status, most of the mercury inside the discharge vessel condense into liquor; only a small proportion of mercury atoms exist in gas status. However, high voltage involves a larger extent of security concerns, but it is hard to generate an electric arc and ignite the lamp if the applied voltage is too low. Besides, the discharge lamps of any variety have a common drawback, that is, under hot lamp status, the lamp needs to be cooled down first before it is ignited again (hot-restrike). That's because gas discharge hardly occurs under a high air pressure. If a lamp which has just been turned off and is still under hot lamp status needs to be used again, the cooling time for hot-restrike exists necessarily to cool down from high temperature to recover a workable air pressure.